Methods of Implementing Low-Power Mode for DSL Modems

ABSTRACT

According to an embodiment, a DSL transceiver is set in a low power mode and moved out of the low power mode responsive to the DSL transceiver receiving data. Data is transmitted only on a first group of sub-carriers when moving the DSL transceiver out of the low power mode, the first group of sub-carriers being a subset of the sub-carriers available to the DSL transceiver for transmission.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/166,458, filed 3 Apr. 2009, which is incorporated herein by referencein its entirety.

BACKGROUND

Multi-carrier transmission systems which provide high speed datacommunication over a local subscriber loop connecting a customer to acentral office are commonly referred to as “xDSL” systems, where “x”specifies a particular variant of DSL (digital subscriber line). Theterm xDSL refers to DSL technologies such as ADSL (asymmetric DSL), HDSL(high bit rate DSL), IDSL (ISDN DSL), SDSL (symmetric DSL), VDSL (veryhigh speed DSL), etc. These and other types of xDSL systems aregenerically referred to herein as “DSL” systems.

In a DSL system, each customer has a modem for communicating with adigital subscriber line access multiplexer (DSLAM) at the central officeof the service provider. The DSLAM terminates and aggregates the DSLcircuits, handing them off onto other networking transports. Eachcommunication channel between a customer and the central office isterminated by a pair of transceivers which communicate with each other.The total bandwidth of the channel interconnecting the customer and thecentral office is typically divided into several different sub-carriers.Each sub-carrier is centered at a particular frequency and has aparticular bandwidth. One group of the sub-carriers is allocated fortransmissions from the central office to the customer modem, i.e. thedownstream direction. A second group of the sub-carriers is allocatedfor transmissions from the customer modem to the central office, i.e.the upstream direction. Additional sub-carriers can be allocated foroverhead and control functions.

Data to be communicated between a customer modem and the central officeis split into groups of bits, one group of bits per sub-carrier. Eachgroup of bits is modulated onto a carrier, e.g. using quadratureamplitude modulation (QAM) and mapped into a vector defined by a pointon the modulation “constellation.” The constellation specifies theallowable data points for transmission. Each point on the constellationis typically referred to as a symbol. The number of bits which ismodulated on each subcarrier is referred to as the bit loading for thissubcarrier. A symbol can represent more bits when a higher-ordermodulation scheme is used or fewer bits when a lower-order modulationscheme is used. During a symbol transmission time period, a symbol istransmitted on each sub-carrier in parallel with the other sub-carriersso that large amounts of data can be transmitted during each symbolperiod.

Conventional DSL equipment provides almost constant data rate for theduration of the link independent of the bandwidth required by thecustomer. However, most customers require high bandwidth only for a fewhours per day. During the remainder of the time, the customerapplications may require only a fraction of the usable bandwidth orpossibly even no bandwidth at all. For example, voice applications suchas VoIP typically require a bandwidth of 128 kbps. Yet, customers whohave signed up only for a voice service have DSL equipment running for24 hours a day without ever using the provided data rate which can rangefrom 256 kbps to 3 Mbps or even higher depending on the type of DSLservice. Maintaining a constant data rate during periods of low or nobandwidth demand unnecessarily wastes power. In addition, existing DSLlines expanded for triple play services (high-speed Internet, TV andvoice) are usually always powered on. The result is an enormous demandof energy for telecommunication equipment, making telecommunicationservice providers some of the single largest energy consumers in theworld.

The ADSL2 standard defines a low power mode (L2 mode). The L2 modeallows modems to reduce the bitloading and/or reduce the transmit powerwhen no data or only a small amount of data is to be transmitted. Inpractice, the ADSL2 L2 mode is not widely used. A significant amount offluctuating crosstalk can occur in a cable binder (i.e., bundle) whenmodems move back and forth between the L2 mode and regular full datatransmission. This fluctuating crosstalk must be accounted for by allmodems coupled to the same cable binder. Otherwise, data errors canoccur. The concept of virtual noise has been introduced to control thenon-stationary crosstalk caused by entering and exiting the ADSL2 L2mode. With virtual noise, modems can be made aware of the DSL lines thatare not active but which can potentially be activated. The modems canuse this information to employ frequency-specific margins for providingprotection against non-stationary crosstalk. However, conventionalvirtual noise techniques cause a significant loss of data rate whenapplied over the complete transmission band. In addition, the ADSL2 L2mode does not allow for fully powering down the sub-carriers, limitingthe power saving potential of the ADSL2 L2 mode. Furthermore, the ADSL2L2 mode is only defined for downstream transmissions. As such, no powerreduction can be realized at the customer side.

SUMMARY

According an embodiment, a DSL transceiver is set in a low power modeand moved out of the low power mode responsive to the DSL transceiverreceiving data. Data is transmitted only on a first group ofsub-carriers when moving the DSL transceiver out of the low power mode,the first group of sub-carriers being a subset of the sub-carriersavailable to the DSL transceiver for transmission.

According to another embodiment, a DSL transceiver is set in a low powermode and subsequently exits from the low power mode. Data is transmittedon a first group of sub-carriers during the exit from the low powermode. A transmit power of sub-carriers not belonging to the first groupof sub-carriers is also gradually increased during the exit from the lowpower mode.

According to yet another embodiment, power consumption by a firsttransceiver is controlled by establishing a communication channel with asecond transceiver over a plurality of sub-carriers using a modulationscheme agreed to by the transceivers and deactivating at least some ofthe sub-carriers responsive to the first transceiver entering a lowpower state. The communication channel is reestablished over a subset ofthe sub-carriers using a predetermined modulation scheme generallyimmune to crosstalk responsive to the first transceiver exiting the lowpower state. The remainder of the sub-carriers are reactivated when morecommunication bandwidth is required than can be provided by the subsetof sub-carriers used to reestablish the communication channel.

According to still another embodiment, power consumption by a DSLtransceiver is controlled by supporting low bandwidth operations at theDSL transceiver using a first group of sub-carriers allocated to the DSLtransceiver while a second group of sub-carriers allocated to the DSLtransceiver is powered-down. The second group of sub-carriers isre-powered so that higher bandwidth operations are supported at the DSLtransceiver using both the first and second groups of sub-carriers.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an embodiment of a DSL systemincluding central office equipment coupled to a plurality of DSL modemsvia a cable binder.

FIG. 2 illustrates an embodiment of a DSL frequency spectrum having agroup of frequency sub-carriers for low power operation.

FIG. 3 illustrates an embodiment of a state transition diagram for DSLequipment configured to implement a multi-tiered power managementscheme.

FIG. 4 illustrates a block diagram of an embodiment of transmit andreceive circuitry of DSL equipment having a power mode controller.

DETAILED DESCRIPTION

FIG. 1 illustrates an embodiment of a DSL system 100 including centraloffice equipment 102 (COE) connected to customer premise equipment 104(CPE) over different communication channels. Each channel is physicallyimplemented over a line 105 bundled with other lines in a cable binder106. Each line 105 may include a twisted wire pair, a fiber optic cableor any other suitable medium for carrying DSL-based signals. The COE 102includes a DSL Access Multiplexer 108 (DSLAM) for aggregatingconnections from many CPEs 104 onto a single, high-capacity connectioncoupled to a network 110 such as an IP or ATM network. Each CPE 104includes a DSL modem 112 for enabling communication with the COE 102over the corresponding channel using any suitable DSL technology. Boththe COE DSLAM 108 and each CPE modem 112 include a power mode controller114. The power mode controllers 114 cooperate with one another toimplement a multi-tiered power management scheme at the CPEs 104 and theCOE 102.

The multi-tiered power management scheme implemented by the power modecontrollers 114 enables each CPE 104 and/or the COE 102 to power down atleast a portion of the resources used to support DSL communication whenhigh bandwidth is not in demand. For example, portions of the CPE modem112 and the COE DSLAM 108 can be powered down when the channel between aparticular one of the CPEs 102 and the COE 104 is inactive or when onlya narrowband service such as Voice-over-IP (VoIP) is supported over thecorresponding channel. Under these conditions, the full bandwidth of thechannel is not needed and thus certain CPE and/or COE resources can bepowered down to save energy. In one embodiment, when additionalbandwidth is subsequently needed, the power mode controllers 114 begingradually re-powering the resources in a controlled manner so that othermodems coupled to the same cable binder 106 are not adversely affectedby the increased crosstalk caused by re-powering the resources. There-powering process can be done in multiple steps so that only the COEand/or CPE resources needed to support a particular bandwidth areadequately powered. The remaining resources can be powered-down to saveenergy since these resources are not needed to meet the currentbandwidth demand. The multi-tiered power management embodimentsdescribed herein can be implemented in the downstream (COE-to-CPE)and/or upstream (CPE-to-COE) directions. Accordingly, energy consumptioncan be reduced as a function of bandwidth demand at the COE 102 and/orthe CPEs 104.

FIG. 2 illustrates an embodiment of a DSL frequency spectrum allocatedfor use by the CPEs 104 coupled to the COE 102 via the same cable binder106. Each CPE 104 is allocated a plurality of sub-carriers for upstreamcommunication and a plurality of sub-carriers for downstreamcommunication. The power mode controller 114 in a particular CPE 104 orthe power mode controller in the COE 102 partitions the frequencyspectrum allocated to that CPE 104 in the upstream and/or downstreamdirections into a first group having a plurality of sub-carriers 200 andat least a second group also having a plurality of sub-carriers 210.Each sub-carrier 200, 210 has a certain bandwidth and is centered at aparticular frequency. Data can be transmitted over the differentsub-carriers 200, 210 in parallel. Some of the sub-carriers 200, 210 ineach group carry information in the upstream direction and othersub-carriers 200, 210 carry information in the downstream direction. Thefirst 116 of sub-carriers 200 is designated as a relatively lowbandwidth “fast wake-up” group meaning that the sub-carriers 200 in thisgroup can be used to quickly reestablish the communication channelbetween the CPE 104 and the COE 102 in either the upstream or downstreamdirection after a low power event subsides. The fast-wake-up group ofsub-carriers 200 may be in one embodiment a band (i.e., a fast-wake-upband) including sub-carriers with successive carrier indexes. Thefast-wake-up group may have in one embodiment a relatively low bandwidthbetween 100 Kbps to 1 Mbps. The term “low power event” as used hereinmeans any event where the full bandwidth of a particular communicationchannel is not needed to reliably support an application implementedbetween the corresponding CPE 104 and the COE 102. A low power event canoccur when there is no activity between the COE 102 and a CPE 104, e.g.after a voice call ends, after a data transaction completes, etc.

Any of the power mode controllers 114 can detect the occurrence of a lowpower event. As such, the multi-tiered power management scheme can beinitiated by either the CPE modem 112 or the COE DSLAM 108. In eithercase, the power supplied to the sub-carriers 210 included in thegroup(s) other than the “fast wake-up” group is reduced in both theupstream and downstream directions during the low power event. This caninvolve lowering the power supplied to the corresponding CPE and/or COEresources or completely deactivating the sub-carriers 210 by shutting ofthe power supplied to the corresponding resources. The sub-carriers 200included in the “fast wake-up” group can also be powered down if desiredas explained in more detail later herein (Step 204). Powering down someor all of the sub-carriers 200, 210 during a low power eventsubstantially reduces power consumption when there is no demand for highbandwidth services.

In one embodiment, a new VoIP call or data transaction may be initiatedat either the COE 102 or one of the CPEs 104. In response, one or bothof the power mode controllers 114 determine whether the sub-carriers 200included in the “fast wake-up” group can provide sufficientcommunication bandwidth after the low power event subsides. The “fastwake-up” group is well-suited for narrowband applications since thegroup has a relatively low bandwidth. For example, if a new VoIPapplication is initiated, the “fast wake-up” group can providesufficient communication bandwidth. As such, the sub-carriers 200included in the “fast wake-up” group are powered-up if previouslypowered-down and the other sub-carriers 210 remain powered down tocontinue saving energy.

Power is gradually increased to at least some of the other band(s) 210when the power mode controller 114 determines that the “fast wake-up”group cannot provide sufficient communication bandwidth after the lowpower event subsides. For example, more bandwidth may be needed when ahigher bandwidth application such as web-surfing or Internet TV islaunched. Each of the powered-up sub-carriers is then used to enablecommunication between the CPE 104 and the COE 102 so that the newapplication can be adequately and reliably serviced. This includes onlythe “fast wake-up” group 200 for low bandwidth applications and one ormore additional sub-carrier group(s) for higher bandwidth applications.

The corresponding channel can be divided into several groups ofsub-carriers. One group is designated the “fast wake-up” group asdescribed above. Each additional group is powered-up and down as neededto support different bandwidth demands. For example, the “fast wake-up”group can be used to service low bandwidth applications such as VoIP. Anadditional group of sub-carriers can be powered-up to support higherbandwidth applications such as voice with pictures. Still another groupof sub-carriers can be powered-up to support even higher bandwidthapplications such as web-surfing. Yet another group of sub-carriers canbe powered-up to support the highest bandwidth applications such as HDTV(high definition TV). The power mode controllers 114 gradually increasethe power provided to additional sub-carriers as bandwidth demandsincrease so that the crosstalk injected into bundled lines included inthe same cable binder changes slowly, enabling equipment coupled tothese lines sufficient time to adjust to the slowly changing crosstalk.This way, services on bundled lines are not adversely affected byfluctuating crosstalk resulting from the multi-tiered power managementembodiments described herein.

The multi-tiered power management scheme implemented by the power modecontrollers 114 provides fast wake-up for voice services. This ensureslow-delay and high quality for voice services, even upon exiting a lowpower state. In one embodiment, voice services can be quickly supportedby the COE 102 and the CPEs 104 in less than 1 second upon exiting a lowpower state. Such a rapid response time is possible by protecting the“fast wake-up” group of all active lines in the binder 106 with virtualnoise. In one embodiment, applying virtual noise to the “fast wake-up”group of all active lines results in a robust modulation or bit-loadingscheme being used for the sub-carriers 200 included in the “fastwake-up” group. Doing so provides sufficient robustness againstfluctuating crosstalk if any other lines of the binder 106 exits the lowpower mode. In one embodiment, data transmitted over the sub-carriers200 of the “fast wake-up” group of the modem 112 exiting the low powermode are modulated using QPSK (quadrature phase-shift keying). This way,the COE 102 and the corresponding CPE 104 need not expend timedetermining a suitable modulation/bit-loading scheme after exiting alow-power state. Instead, a robust modulation/bit-loading scheme isready for use without having to observe channel conditions, etc. Thisenables the COE DSLAM 108 and the corresponding CPE modem 112 to quicklyreestablish the communication channel using the “fast wake-up” group ofsub-carriers 200 upon exiting a low power state. In addition, themulti-tiered power management scheme implemented by the power modecontrollers 114 is compatible with any DMT-based (discrete multi-tone)DSL technologies such as ADSL2, ADSL2+, VDSL2, etc. in both the upstreamand downstream directions where DMT is a form of multi carriermodulation and as applied to ADSL, the frequency spectrum is made up ofbetween 0 Hz and 1,104 MHz divided into 256 distinct sub-carriersseparated by 4.3125 MHz.

FIG. 3 illustrates an embodiment of a state diagram corresponding to theoperation of the CPE modem 112, the COE DSLAM 108 and the power modecontrollers 114. The state diagram includes several conventionaloperational states such as initialization (300), full data transmission(302) and power down (304) in which the CPE modem 112 and the COE DSLAM108 can operate. These states are well understood to one of averageskill in the DSL communication arts, and thus no further description isprovided. FIG. 3 also illustrates additional states associated with themulti-tiered power management scheme implemented by the CPE modem 112and the COE DSLAM 108. These states are described next with reference tothe power mode controller 114 of the uppermost CPE modem 112 shown inFIG. 1 for ease of explanation only. However, the power mode controller114 included in the COE DSLAM 108 and the other CPEs 104 can alsosupport the same states. Thus, FIG. 3 represents the operational statesof both each CPE modem 112 and the COE DSLAM 108.

If the CPE modem 112 is not transmitting any payload data, the powermode controller 114 detects the low-power event and moves the modem 112into the low power mode (306). In this mode, the power mode controller114 can either reduce the transmit power or completely stop thetransmission by deactivating most of the sub-carriers allocated to themodem 112 and the corresponding modem resources. According to thisembodiment, the modem 112 maintains a pilot carrier and/or a fewadditional signaling carriers with the COE 102, e.g. over some of thesub-carriers 200 included in the “fast wake-up” group. When thelow-power event subsides, the modem 112 quickly begins a transmissionusing the “fast wake-up” group of sub-carriers 200. This transitions themodem 112 to the first wake-up phase (308). Initially, the modem 112 canuse a robust modulation scheme such as QPSK in the “fast wake-up” group.The training for the “fast wake-up” group can be achieved with a veryshort training sequence, e.g. less than 1000 DMT symbols. Accordingly,if the modem 112 is in the power down mode (304) and is only observingthe fast wake-up group of sub-carriers 200, data transmission can eitherstart directly by using a default transmission parameter (e.g., adefault bitloading or modulation scheme) or a very short training isneeded when a signal is detected in the “fast wake-up” group. The newchannel can be used for low bandwidth applications such as a voice call.If such a low bandwidth application needs to be transmitted, then noadditional training is required and the modem 112 can go back into thelow power mode (306) at the end of the application. When higherbandwidth is required, e.g. additional services/applications requiremore bandwidth, the modem 112 initiates a second wake-up phase (310).

In the second wake-up phase (310), the bit-loading for the “fastwake-up” group of sub-carriers 200 is increased to a higher value and/orthe order of the modulation is increased to accommodate a higherbandwidth. The new bit-loading value and/or modulation scheme preferablytakes into account the virtual noise definition specified for the “fastwake-up” group so that the CPE 104 moving out of the low power state isnot adversely affected at a later point in time by any other modem 112exiting the low power mode in the same binder 106. The new bit-loadingvalue and/or modulation scheme can be obtained from a stored table 116which was used in previous connections or can be based on an SNR(signal-to-noise) measurement performed on the robust modulationchannel. According to this embodiment, the SNR is measured on thesub-carriers belonging to the “fast wake-up” group of sub-carriers and abitloading is determined based on the SNR measurement. The modem 112 andthe COE DSLAM 108 exchange the new bit loading value and/or modulationscheme so that the bandwidth of the corresponding communication channelcan be increased. One or more additional properties associated with thedata transmission can also be exchanged between the modem 112 and theCOE DSLAM 108.

In a third phase (312), the modem 112 communicates a request to the COEDSLAM 108 for using the remaining group(s) of sub-carriers 210. Therequest is made by messaging over the trained “fast wake-up” group.Following approval of the request, the power mode controller 114gradually increases the transmit power in the remaining group(s) ofsub-carriers 210 and begins retraining the modem 112 accordingly. Theincrease in transmit power is slow enough to allow other modems coupledto the same binder 106 to take the effect of increased crosstalk intoaccount, e.g. by adjusting their bit-loading and/or modulation usingonline reconfiguration techniques such as seamless rate adaptation orbit-swapping. Gradually increasing the sub-carrier transmit power inthis way ensures that other lines in the same binder 106 are notadversely disturbed by the fluctuating crosstalk generated from the linewhich is exiting the low power mode (306). In some embodiments, thesecond and third phases (310, 312) could be performed at the same pointin time to reduce the wake-up time. In addition, the third phase (312)can be further divided into multiple steps to make an even smoothertransition to full power mode (302). Other state transitions are alsopossible. In one embodiment, the CPE modem 112 and/or COE DSLAM 108 canmove from the full data transmission phase (302) directly to either thefirst wake-up phase (308) or to the second wake-up phase (310), e.g.when the bandwidth demand of the current application(s) falls below acertain threshold. In another embodiment, the CPE modem 112 and/or COEDSLAM 108 can move from the power-down mode (304) directly to thelow-power mode (306) by observing only the fast wake-up group ofsub-carriers 200 when in the power-down mode (304) as previouslydescribed herein.

The “fast wake-up” group implemented by each CPE modem 112 and/or theCOE DSLAM 108 is located in a frequency band receivable for thecorresponding CPE modems 112 in the same binder 106, or at least for themodems 112 that support the low power mode (306). The “fast wake-up”group is preferably located at low frequencies. The bandwidth and thelocation of the “fast wake-up” group can be determined by the serviceprovider. The bandwidth is preferably wide enough to provide sufficientbandwidth for a voice call with a robust modulation scheme such as QPSK.For example, a voice call transmitted with 128 kbps would require 16QPSK modulated sub-carriers assuming a DMT symbol rate of 4 kHz (notincluding framing overhead). The “fast wake-up” group can be implementedby more than one continuous DMT frequency band, but can also be formedof multiple non-continuous frequency bands. According to embodiments,the “fast wake-up” group includes lower frequency sub-carriers. For theADSL2/ADSL2+ and long reach VDSL2 standards, the upstream group isallocated up to 138 kHz/276 kHz and is commonly referred to as the“upstream band-0” or “extended upstream band-0.” In embodiments, onlysub-carriers of the upstream band-0 are used to support the “fastwake-up” group. The upstream band-0 has a limited number of carriers andcorresponding sub-carriers. However, the upstream band-0 is located atvery low frequencies. At such frequencies, crosstalk is not as strong asat higher frequencies. Therefore, the virtual noise protection given tothe “fast wake-up” group by the power mode controller 114 can be limitedand results in only a negligible data rate loss over the “fast wake-up”group.

The multi-tiered power management scheme can be implemented by the powermode controllers 114 in hardware, firmware or some combination of both.Those skilled in the art can implement various portions of thedescription, block diagrams and operational flows described herein inthe form of computer-executable instructions, which may be embedded inone or more forms of computer-readable media. As used herein,computer-readable media may be any media that can store or embodyinformation that is encoded in a form that can be accessed andunderstood by a computing device. Typical forms of computer-readablemedia include, without limitation, both volatile and nonvolatile memory,data storage devices, including removable and/or non-removable media,and communication media. Accordingly, existing CPE modems can implementmulti-tiered power management scheme without requiring a redesign. Forthose modems not capable of implementing the multi-tiered powermanagement scheme, these modems may protect themselves againstfluctuating crosstalk by defining appropriate virtual noise levels, i.e.the “fast wake-up” group of sub-carriers is protected with a sufficienthigh noise margin. For additional power savings when operating in the“fast wake-up” group of sub-carriers, additional dedicated hardware canbe provided to support this operational mode. The hardware may includeonly those circuits needed to support the multi-tiered power managementscheme, thereby reducing hardware power consumption during low-powerevents. For full data rate transmission, additional hardware can bepowered-up to accommodate higher bandwidth demand.

FIG. 4 illustrates an embodiment of the front end portion of the CPEmodem 112 and COE DSLAM 108, including the power mode controller 114.According to this embodiment, the front end includes transmit andreceive circuitry. The transmit circuitry includes a framer 402 formultiplexing serial data into frames, generating FEC (forward errorcorrection) and interleaving the data. An encoder 404 encodes the framesto produce constellation data. For low bandwidth transmissions over the“fast wake-up” group of sub-carriers, the encoder 404 assigns a numberof bits per tone corresponding to the robust bit-loading/modulationscheme predetermined for the “fast wake-up” group as previouslydescribed herein. For higher bandwidth transmissions, the encoder 404assigns the maximum number of bits per tone, e.g. based on measured SNRof each carrier and generates a QAM (quadrature amplitude modulation)constellation where each point represents a digital value. Eachconstellation point can be one of N complex numbers each having distinctphase and amplitude components.

The output of the encoder 404 is input to a DMT modulator 406 whichgroups N constellation points to a vector. The vector of N constellationpoints is input to an IFFT (inverse fast Fourier transform) module 408which duplicates each carrier with its conjugate counterpart so the 2Noutput samples are real. The 2N time domain samples are extended by acyclic extension consisting of cyclic prefix and optionally a cyclicsuffix, and are then filtered 410 and input to a DAC 412(digital-to-analog converter). The set of time domain samples representsa summation of all the modulated sub-carriers, for the duration of onedata frame. The DAC 412 converts the digital transmit bit stream toanalog form. The output of the DAC 412 is filtered by an analog filter414 and amplified by a line driver 416. The line driver 416 interfacesto the line via a hybrid circuit (not shown).

On the receive side, downstream signals are filtered by a receive filter418 and converted to digital form by an ADC 420 (analog-to-digitalconverter). A signal detector module 422 identifies a wake-up signal,e.g. based on a received signal strength measurement. The signaldetector module 422 also enables “wake-up” of other necessary elements,including the power mode controller 114 if powered down in the low powermode (306). The signal detector module 422 can avoid false wake-upscaused by power received from crosstalk or other noise sources. Theoutput of the signal detector 422 is filtered by digital filters 424. AnFFT (fast Fourier transform) module 426 transforms the carriers back tophase and amplitude information, e.g. N complex QAM symbols which arethen processed by a demodulator 428. The demodulator 428 outputsestimates of received symbols and can correct for attenuation of thesignal amplitude and phase shifts. A decoder 430 converts the symbolestimates into frames which are then de-multiplexed into a serial bitstream by a deframer 432.

The power mode controller 114 can power down any of the resources whennot needed to accommodate the current bandwidth demand. In oneembodiment, the power mode controller 114 resizes the IFFT and FFTmodules 408, 426. In another embodiment, the power mode controller 114changes the resolution of the DAC and ADC 412, 420. In yet anotherembodiment, the power mode controller 114 reduces the supply voltageapplied to the line driver 416. The power mode controller 114 can alsochoose a different bitloading and/or modulation scheme that enables thelow power mode (306), e.g. reduced-state bitloading and/or low bit ratemodulation over a narrow band. The power mode controller 114 recognizeswhen the modem 112 should come out of low power mode and configures theappropriate changes. The power mode controller 112 can also reconfigurethe modem 112 for transmitting and receiving a pilot tone for enablingclock synchronization. Alternatively, the modem 112 has a plurality oftransmit and receive paths and the power mode controller 114 can switchbetween one or more transmitter paths used only for transmitting signalsin the low power mode (306) and receiver paths used only for receivingsignals in low power mode (306).

During operation, group information and virtual noise parameters for theupstream and/or downstream “fast wake-up” group of sub-carriers arecommunicated to the CPE modem 112. The modem 112 applies virtual noiseto protect the “fast wake-up” group of sub-carriers as previouslydescribed herein. The modem 112 also performs the remainder of thestandard DSL initialization (300). The modem 112 performs standard fulldata rate transmissions without modification (302). During a low-powerevent, the power mode controller 114 transitions the modem 112 to thelow power mode (306). In one embodiment, the modem 112 uses the standardADSL2 L2 mode to signal the COE 104 to move into the low power mode.According to one embodiment, only a pilot carrier is transmitteddownstream if there is no payload data to be transmitted in the lowpower mode (306). Either a CPE 104 or the COE 102 decides to exit thelow power mode (306), e.g. responsive to a signal received from theservice provider or by monitoring transmit/receive buffer capacity. Inresponse, a training signal is transmitted in the assigned “fastwake-up” group of sub-carriers. Upon detection of the signal, the otherside also transmits a training signal in its assigned “fast wake-up”group of sub-carriers. After either a fixed duration or aftertransmission of some markers, e.g. SEGUE as defined in ADSL2 or SYNCHROsignals as defined in VDSL2, the modem 112 starts data transmission witha robust modulation scheme such as QPSK (308). Because a robustmodulation is used in the first wake-up phase (308), a power back-offcan be transmitted to reduce the effect of the crosstalk on the otherlines in the same binder. The first wake-up phase (308) is sufficientfor transmitting low bandwidth data such as a voice call.

Either the COE 102 or a CPE 104 signals the other side with controlmessages if additional bandwidth is needed or if the data rate of thefirst phase (308) is sufficient. If additional bandwidth is required,then a new bit-loading value and/or modulation scheme is applied to thesub-carriers in the “fast wake-up” group and the transmit power in thegroup is increased in the second wake-up phase (310). The newbit-loading value/modulation scheme can be selected from the storedtables 116 or based on a SNR measurement as previously described herein.The new bit-loading/modulation information is preferably exchangedduring the first wake-up phase (308).

Preparing the modem 112 for moving back to the full data transmissionphase (302) is done in the third wake-up phase (312). In this phase(312), the transmit power in the remaining groups(s) of sub-carriers isgradually increased. Robust training signals can be initiallytransmitted and only after the full transmit power is reached is fulldata transmission started. The transmit power increase could be done inseveral steps which can be predetermined or programmed by the serviceprovider. In addition, there can be more defined wake-up phases andsub-carrier groups. This is advantageous for servicing multipleapplications having different bandwidth demands such as voice, internetradio, HDTV, etc. Preferably, there is a dedicated wake-up phase andfrequency sub-carrier group with a specific maximum bandwidth for eachapplication class. The power mode controller 114 chooses the minimumwake-up phase that fulfills the bandwidth demand of the applicationclass currently requested. After completion of the task, the CPE 104and/or the COE 102 can return to the low power mode (306).

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper”, and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first”, “second”, and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

1. A method comprising: setting a DSL transceiver in a low power mode;moving the DSL transceiver out of the low power mode responsive to theDSL transceiver receiving data; and transmitting data only on a firstgroup of sub-carriers when moving the DSL transceiver out of the lowpower mode, the first group of sub-carriers being a subset of thesub-carriers available to the DSL transceiver for transmission.
 2. Themethod of claim 1, comprising transmitting the data with a robustmodulation on all sub-carriers of the first group of sub-carriers. 3.The method of claim 2, wherein the robust modulation is a QPSKmodulation.
 4. The method of claim 1, comprising transmitting the dataon the first group of sub-carriers with a reduced transmit powerspectrum density compared to a power spectrum density during a datatransmission in a full power mode.
 5. The method of claim 1, comprising:initially transmitting training signals when moving the DSL transceiverout of the low power mode; and thereafter, transmitting the data whenmoving the DSL transceiver out of the low power mode.
 6. The method ofclaim 1, comprising: performing a SNR measurement on the first group ofsub-carriers; and exchanging bitloading information between the DSLtransceiver and a further DSL transceiver.
 7. The method of claim 1,comprising moving the DSL transceiver back to the low power mode when nofurther data is to be transmitted.
 8. The method of claim 1, comprisingstoring information indicating at least one property of the transmissionduring the moving out of the low power mode.
 9. A method comprising:setting a DSL transceiver in a low power mode; subsequently exiting theDSL transceiver from the low power mode; transmitting data on a firstgroup of sub-carriers during the exit from the low power mode; andgradually increasing a transmit power of sub-carriers not belonging tothe first group of sub-carriers during the exit from the low power mode.10. The method of claim 9, comprising initially transmitting trainingsignals on the sub-carriers not belonging to the first group ofsub-carriers during the exit from the low power mode.
 11. The method ofclaim 9, comprising: measuring a SNR on the sub-carriers not belongingto the first group of sub-carriers; determining a bitloading based onthe SNR measurement; and exchanging the bitloading information.
 12. Themethod of claim 11, comprising transmitting the bitloading informationon the sub-carriers of the first group of sub-carriers.
 13. The methodof claim 12, comprising transmitting in addition to the bitloadinginformation other information on the sub-carriers of the first group ofsub-carriers, the other information indicating at least one furtherproperty of the data transmission.
 14. The method of claim 9, comprisingincreasing the transmit power on the sub-carriers not belonging to thefirst group only in one transmission direction during the exit from thelow power mode.
 15. A method of controlling power consumption by a firsttransceiver, comprising: establishing a communication channel with asecond transceiver over a plurality of sub-carriers using a modulationscheme agreed to by the transceivers; deactivating at least some of thesub-carriers responsive to the first transceiver entering a low powerstate; reestablishing the communication channel over a subset of thesub-carriers using a predetermined modulation scheme generally immune tocrosstalk responsive to the first transceiver exiting the low powerstate; and reactivating the remainder of the sub-carriers when morecommunication bandwidth is required than can be provided by the subsetof sub-carriers used to reestablish the communication channel.
 16. Themethod of claim 15, comprising selecting the subset of sub-carriers forreestablishing the communication channel so that the sub-carriersincluded in the subset provide a collective bandwidth of between 100Kbps and 1 Mbps.
 17. The method of claim 15, comprising exiting the lowpower state and reestablishing the communication channel over the subsetof sub-carriers responsive to a new VoIP call initiated at the first orsecond transceiver.
 18. The method of claim 17, comprising reactivatingthe remainder of the sub-carriers responsive to an application initiatedat the first or second transceiver requiring more bandwidth than theVoIP call.
 19. A method of controlling power consumption by a DSLtransceiver, comprising: supporting low bandwidth operations at the DSLtransceiver using a first group of sub-carriers allocated to the DSLtransceiver while a second group of sub-carriers allocated to the DSLtransceiver is powered-down; and re-powering the second group ofsub-carriers so that higher bandwidth operations are supported at theDSL transceiver using both the first and second groups of sub-carriers.20. The method of claim 19, comprising gradually re-powering the secondgroup of sub-carriers so that modems communicating over other linesincluded in the same cable binder coupled to the DSL transceiver canadjust respective bit-loading schemes implemented at the modems foradapting to crosstalk caused by re-powering the second group ofsub-carriers at the DSL transceiver.